Acoustic device



ug. 9, 1938. R, N MARSHALL y 2,126,566

ACOUSTIC DEVICE Filed D60. 29) 1934 5 Sheets-Sheet 2 /N VENT @MMA/95124'BV @am 6,. 1M

' A TTORNEY Aug. 9, 1938. R. N. MARSHALL' u I 2,126,566

ACOUSTIC DEVICE Filed Deo. 29. 1934 5 Sheets-Sheet 4 /NVENTOR By.RNMARSHALL @am 5. 141% ,i

A TTORNEY R. N. MARSHALL 2,126,566

ACOUSTIC DEVICE Filed Dec. 29. 1934 5 Sheets-Sheet 5 maan, 6, Map2.

ATTORNEY Patent-ed Aug. 9,1938

UNITED STATES PATENT I oFFicE y Application netsnoer ze, 1934, serai No.'15am 17 Claims.

I'his invention relates to acoustic devices; and, more particularly, tomicrophones or transmitters of the moving coil or electrodynamic type.

An object of this invention is to improve the structure and operatingcharacteristics of acousl means for enhancing the low frequency responseof the device.

Still another feature comprises an enclosure or case for an acousticdevice in which the case interior is divided into at least two chambersconv nected by a passage that may be open or closed at the will of theuser to control the effective volume of the case.

' Other and further features will be evident from the detaileddescription which follows hereinafter.

In accordance with this invention, an acoustic device comprises a caseopen at its ends of different sizes, and enclosing a devicecomprislng adiaphragm, moving coil, and magnet structure,

3c the diaphragm being in alignment with and closing the ysmaller end ofthe case. The larger end of the case is closed by a plate or wallmember, supporting the magnet structure and. diaphragm in position, andvcontaining a passage. A cover member forming part of the case is securedtothe remaining lportion of the case by a plurality of plug pins, eachprojecting at one end from the 'l cover and threadably engaging the wallmember at the other end. 4The cover and wall members denne another airchamber in which is positioned an elongated, open-ended tubular member,one open end of which is secured to the cover and in alignment. with an.apertured portion, thereof. -lf desired, the tubular member may replacedby an elongated groove in the case, either on the inner or outer surfacethereof, a closure member overlying vthe groove, the closure member andcase containing passages connecting the interior of the case and' theatmosphere through the :30V groove.' The closure member may beadjustable in position to vary the effective length of the i groove. Thepassage connecting the chambers in the case may be covered withanacoustic resistance material, means being provided to vary the 'I 55effective area thereof,

A more complete understanding of this invention will be derived from thedetailed description which follows, taken in conjunction with theappended drawings, wherein: l

Fig. 1 is an enlarged cross-sectional view of anx 5 acoustic deviceconstructed in accordance with this invention;

. Fig. 2 is a cross-sectional view of the device of Fig. 1, taken alongthe line 2-2 thereof;

Fig. 3 is-an end view of the device of Fig. 1, 10 taken along the line6-3 thereof;

Fig. 4 is a view of the cover orclosure member of the device of Fig. 1,taken along the line 4--6 thereof:

Fig. 5 is a sideelevation, substantially actual 15 size, of the deviceof Figs. 1-4, a jack or connector being shown associated with it; Y Fig.6 is an enlarged cross-sectional view of another acoustic deviceconstructed in accordance with this invention; 20

Fig. 'l is an end view of the device of Fig. 6, taken along the line 1-1thereof with the cover or closure member removed, and partially brokenaway to show the cord; Y

Fig. 8 is a cross-sectional view, apprczdmately 25 actual size, of thedevice of Fig. 6, taken along the line 6-6 thereof;

Fig. 9 is an enlarged, cross-sectional view of still another acousticdevice constructed in accordance with this invention;

Fig. 10 is a cross-sectional view, approximately actual size, of thedevice of Fig. 9,y taken along the line IO-ll thereof; f A

Fig. 11 is a front viewsubstantially actual size, of another acousticdevice in accordance with 35 this invention; i A

Fig. 12 is a side elevational view of the device of Fig. 11;

Fig. 13 is an enlarged, cross-sectional view of me device of ng.11,1:axen along me une n n 40 thereof, and with the front screen andassociated frame removed; y

Fig. 14 is'a cross-sectional view of the device of Figs. 11 l3, taken'along the line II-Il of Fig. 13; and

Fig. v15 is a plan view of afragment of the device of Fig. 11. A

The acoustic device of l'lgs.' 1-5 comprises an electrodynamicmicrophone or transmitter, designated generally, Il. -It comprises ahollow housing, container, enclosure or case Il, of insulating material,for instance, phenol linen, inwardly curved'to present a substantiallyhemispherieal outer surface at its front end 22, which containsacircularopening inwhichanaperturcdplate 5';

or protective grid 2|, for instance, of brass, is positioned. Theinterior of the case is divided into two air chambers or compartments2l, 2B by a disc, plate or partition member 21, held against theinternally projecting shoulder or flange 2l by a lock or clamping ring2l, externally threaded for engagement with the internally threadedannulus Il oi' the case.

An electrodynamic unit 2| ls mounted in one chamber 2i, and comprises aUshaped permanent magnet 32, preferably of cobalt steel orcobait-molybdenum steel; an outer pole member 3l, preferably of magneticiron; an inner or central and substantially cylindrical pole member 24.preferably of cobalt steel or cobalt-molybdenum steel; and a dome-shapedor cap member l5, preferably of magnetic iron. The outer pole has anannular, central portion 28, and a pair of radially outwardly,downwardly projecting extensions 21, Il, one extension 31 containing acircular passage II, and the other a slot Il ex tending inwardly fromits periphery, the passage Y and slot receiving pin members 4|, forinstance,

of steel, projecting from the ends 42 of the magnet. Pole member 24 hasa central passage '4I for a clamping member or bolt Il, the central poleand the cap member being fastened to the central and straight portion 45of the permanent magnet by this bolt, the threaded end I6 of which isengaged by a pair of nuts l1.

'111e diaphragm 4I is of any suitable material, for instance, alight-weight, high-strength material, such as duralumin. It comprises acentrally stiiiened, dome-shaped portion 49; an outer, annularperipheral portion Si; and an-in termediate, flexible,tangentially'corrugated portion Il. The annular portion is secured tothe annular ridge or flange $2 of the outer pole, preferably by cement,and such that the coil 53, which may be of edge-wound, insulated,aluminum ribbon or tape and secured to the diaphragm at the junction Ilof the portions 49 and 5| thereof, is centered in the annular air-gapdefined by the members 22, 35. The coil leads (not shown) are connectedto terminal members (not shown) carried by arcuate-shaped terminalstrips Il of insulating material, that are fastened to the inner surface5l of the pole member 23.

Acoustic damping means is positioned under the air-gap and comprises anannulus 58 of damping cloth, preferably silk. It is supported by a plateor ring member l! of non-magnetic material, for instance, brass,containing a pluralityof elongated passages 8|. The ring 59 ismaintained in position by a resilient sleeve or ring Il, preferably ofsoft rubber, that encircles the central pole and is carried by the ringmember Il supported on the shoulder or ledge I2, and ls spaced from theunder surface il of the outer pole by a resilient washer or ring Il,preferably of soft rubber.

The diaphragm end of the unit llcloses the case opening 23, a resilientwasher il, for instance, rubber, being interposed between the outerportion of the diaphragm and the radially extending nange portion toprovide an air proof seal. It is apparent that the electrodynamic unit2| is compact and may be .mounted in a confined and substantiallyhemispherical space in such a manner as to utilize the available spaceto the greatest extent possible.

The partition member 21 is of insulating material and contains afrusta-conical shaped opening or passage 01 connecting the chambers 2l,2i,

the smaller end opening into chamber 2B. It has embedded in it a pair ofinternally threaded bushings 8l, and a cup-shaped member 69, each ofwhich may be of brass. The cup member contains a helical spring 10, forinstance, of phosphor bronze. The clamping nuts 41 bear against thespring 10, which with the partition member in place, is undercompression, whereby the unit 3| is forced forward to cause thediaphragm outer portion to bear against the washer E5. The bushings 62are electrically connected by suitable conductors (not shown) with thecoil lead terminals on the members 5B.

The second chamber 26 is defined by the partition member and a removableportion or cover 1| of the case. This cover supports on its innersurface, an elongated, coiled tubular member 72, open at each end, andone end 'i3 of which is secured to the cover by a clamping plate 14,screws 92 and gasket l5, in line with the multiaperture portion l!thereof. This tubular member is provided in accordance with theteachings of A. L. Thuras Patent 1,869,178, issued July 26, 1932, and ina device constructed in Vaccordance with Figs, 1-5 was approximately sixinches long and one-eighth inch in inner diameter.

The cover contains an elongated slot or recess l1 adapted to receivesjack 'i8 for engagement with three pins or contacts 19 which aresupported by the case and project into the slot. The pins have threadedextremities all, which thread into the bushings and cup member to securethe cover to the remaining portion of the case. Where the pins extendfrom the cover into the slot, they are surrounded by a suitable sealingcompound Il, for instance, sealing wax. A tight seal between the coverand the rest of the case is ensured by providing the cover withsharp-edged annular ridge l2, and inserting between it and the shoulderportion 83 of the case, a resilient ring I4 adapted to be compressedwhen the cover is united to the rest of the case.

The chambers 25, 26 and the passage El permit wave propagation from therear of the diaphragm when the microphone is being used. The tubularmember, of course, causes the response of the microphone to be enhancedin the region of the low sound wave frequencies.

Supported on the case and in front of the grid 2Q, is a screencomprising a double layer 85 of cloth, preferably, silk. It is mountedbetween a pair of protective brass screens I6, within a channeled ringl1, together with a ring member Il having three equally spacedprojecting portional! having rivets Sl at their extremities forengagement with three equally spaced recesses S2 in the case. I'hisscreen is provided in accordance with the teachings of the pendingapplication of F. F. Romanow, Serial No. 729,759, filed June 9, 1934,Patent No. 2,106,813, issued Feb. 1, 1938, to render the open fieldresponse characteristic of the microphone substantially uniformregardless of the microphones position with reference to the sound wavesource.

The acoustic device of Figs. 6, 7 and 8 comprises an electrodynamicmicrophone or transmitter, designated generally, |00. It comprises .ahollow housing, container, inclosure or case Illl, the interior of whichis divided into two air compartments or chambers |02, lll by a disc,plate or partition member IM containing a frusto-conically shapedpassage or opening |05 Join ing the two chambers. With such a shapedpassage, the acoustic mass effect of the air in the passage issubstantially negligible. 'Ihe chamtion or cover member |05 of the case.The casev is curved at one end |01 so as to present a substantiallyvhemispherical outer surface, and contains a circular opening 20 inwhich the grid 24 is positioned. The electrodynamic u'nit |03 issubstantially the same as the unit like parts bearing like identifyingnumerals. The magnet 45, pole member 34 and dome-shaped or 'cappole-piece |00 are secured together by a screw ||0, the head of whichnts into the central aperture ||2 in the partition member. A sealingwasher ||0 is interposed between the magnet and partition member. Theunit is maintained in position by the partition member, which, in turn,is held in place by a lock or clamping rin'g ||4, externally threadedfor engagement with the threaded annulus |I of the case. Y

The case contains a circumferentially arranged groove, channel or receas||5 in its outer surface, a closure member or ring ||1 being disposedthereover, having a sliding t with the annular surface portion ||0 ofthe case, and resting against the shoulder or ledge portion H9. The

member ||1 contains an opening or aperture |20 (see Fig. 8) connectingthe atmosphere outside of the case with the groove; the groove at oneend connects with the interior of the case, speeically, chamber |02,through a passage or opening |2|. These apertures |20,' |2| are circularand of approximately the same area as the crosssectional area of thechannel or groove ||5. The

member ||1 is rotatable such that the position of the aperture |20 withreference to the aperture |2| is variable to varylthe effective lengthof the tubular passage. If the member ||1 is adjusted so that theaperture |20 is positioned over the portion |22 of the case, theinterior of the case is closed, of course, from the atmosphere outsidethe case. This groove or channel functions,

like the tubular member 12, to enhance the low frequency response of theacousticdevice and, since its effective length is variable, the low frevquencyresponse may be varied at will.

It may be desired to modify the wave propagation in the chambers |02,|00. Closure means |20 is provided for closing the passage |05, and byisolating chamber |03, to reduce lthe eil'ective volume of air on therear side of the diaphragm. It comprises a pad |24 of'resilientmaterial, such as rubber, between a pair of washers |25, and fastened bya bolt |25 and nut |21 to one end of a wire frame member |23, the otherend of which is supported Vin vslideways |20 of a U- shaped frame member|00 securedto the partltion member by any suitable means, for instance,rivets |0|. A helicalspring |02 is connected at one end to the pad endof the member |20, and at its other end to the lug portion |00 of themember |00. The angularly projecting end portion |04 of the member |20is engaged by one end of a link |05 whose other end |05 projects throughan opening |01 in the case wall and terminates in a button member' |03adapted tol indicated by the broken lines.- A' slight pressureonanebutton lss returns the closure means u; its original position underthe'. restoring "action frame member |00. The latter is engaged bythehead of the bolt H0. The terminals |4| 'are electrically connected bysuitable conductors (not shown) with the terminal members on theinsulators 55.

The devices of Figs. 6, '1 and 8 and Figs. 9 and are substantiallyalike, like parts bearing likereference numerals, that of Figs. 9 and10, however, embodying a different arrangement of the low sound wavefrequencies enhancing means. The inner surface |50 of the case |5|contains a groove, channel or recess |52 over which is positioned aring-shaped closure member |50, preferablysecured to the surface |50 bya screw |54. 'I'he case contains an aperture or passage|55 at one end ofthe groove connecting it with the atmosphere outside the case, and theclosure member contains an aperture or passage |56 at the other end ofthe groove connecting 'the' latter withv the interior of the casing,specifically, chamber |02. 'The passages |55, |55 are preferablycircular and of approximately the same area as lthe cross-sectional areaof the groove.

- threaded for engagement. with the threaded por- A removable portionor.

tion |51 of the case.'l cover |55 defines, in part, the chamber |53'.The case outer surface contains a groove |59, similar to the groove ||5ofthe device of Figs. .6-8, a ring-shaped closure member |10 beingslidably positioned thereover and bearing against the shoulder or ledge|1| of the case. A retaining member orl screw |12 prevents the closuren-iem-v ff ber from shifting toward the front of the transmitter. Theclosure member contains an aperture 20| similar to the aperture |20 ofthe closure member H1; the aperture or passage |13 connectsthe groove|59 and the interior of the case, specifically, chamber |52. To ensurethat the closure member .will remain in its adjusted position, one edgeis provided with spaced notches |14 adapted tovbe engaged by thestepped-end |15 of a finger-operable rod |15., This rod extends -througha passage |11 in the upperpart of the case. and is encircled for aportion of its length by a helical spring |18-, normally undercompression. Normally, therefore, the ,rod cooperates with the closuremember. When it is desired to adjust the position of the aperture in theclosure ,member with reference to the aperture leading from the'grooveto the case interior and, thereby, to vary the effective length of thepath between the exterior -and interior of the case, the nger engageablemember |19 on'the rod |15 is pushed rearwardly to move the end of th'erod out of the notch against the restoring action of the spring.Theclosure member is then turned to the desired degree, the pressureon'the member |10 isreleased, andthe rod engages again with a notch-inthe closure member.

'7'5 of thespring. A three conductor cord |00 ex- The partition membercontains a plurality of 75 spaced passages |88, preferably of lessdiameter than the thickness of the partition member and connecting theair chambers |52, |83. A strip |8| of damping material, for instance,silk, is secured to one surface |82 of the partition member and acrossthese passages. on the other surface of member |88`and is adapted to beadjusted in position manually from outside the case by means of a rod|84, so that some or all of the passages |88 are obstructed. Not only isthe effective volume of air on the rear or inner side of the diaphragmthus controllable, but the case resonance is eliminated by the provisionof the right number of passages or holes |88 and/or damping materialill. In small microphone design, in which large cases in comparison withthe size of the electrodynamic unit are required, uncontrolled caseresonance has been found undesirable.

The opening 28 is closed by a multi-apertured plate or grid I 85 whichfills the cavity in front of the diaphragmand increases the normal rangeof the electrodynamic unit at high frequencies by preventing resonanceby the air in the opening 28 until, for example, about 12,00015,000cycles per second, or by leaving unaffected the normal reso nant effectof the air of the opening, but adding another resonance at a higherfrequency. 'I'he passages or holes |88 in the grid are shown of greaterlength than diameter although they may be of greater diameter thanlength, and, since the effect of a particular grid is critical with itsdistance from the diaphragm, the high end response of the transmittermay be made variable by providing the grid with an externally threadedportion |81 for engagement with the internally threaded portion |88 ofthe case.

Supported on the case |8| and in front of the grid |88, is a framemember|88. It comprises an apertured plate member |88 behind which a strip ofdamping cloth, silk (not shown), is positioned in accordance with thedisclosure ofthe pplication of F. F. Romanow, Serial No. 729, 59, filedJune 9, 1934, Patent No. 2,106,813, issued Feb. 1, 1938. lIhe member |80is integral with a plurality of spaced supporting arms |8I, integral at'their other ends with a ring member |82 which contains an elongated slot|88. A guide member Vor screw |88 projects from the case, specifically,from the closure member |18. When it is desired to adjust the positionof the cloth screen with reference to the case, the screw is loosened,the frame member turned with the screw acting as a guide in the slot,and the screw tightened to fasten it in its new position.

While the featuresrQt-jthis invention have been disclosed with referenceto several specic embodiments, it is to be understood, of course, thatvarious modifications may be made therein without departing from thescope of this invention as defined in the appended claims.

What is claimed is: f

1. An acoustic device comprising a case having anopen end, a sound wavetranslating unit in said case and including a diaphragm and a magnet,said diaphragm closing said open end, means in said case between saidmagnet and the other end of said case dividing it into two chambers andbearing against said magnet to hold it in position so that the diaphragmcloses said case open end, said means containing a restricted. smallarea passage connecting said chambers, and means constituting arestricted sound wave passage for enhancing the low frequency responseof the de# A slide |83 is supported vice, connecting one of saidchambers with the atmosphere outside of the case.

2. An acoustic device comprising a case having an open end, a diaphragmin said case and closing said open end, and means supported by said caseand defining with the case a variable length,

restricted sound wave passage connecting the in-` terior of the case andthe atmosphere, for enhancing the low frequency response of saiddiaphragm.

whereby the distance between said passages may be varied.

4. A n acoustic device comprising a case having Van open end, adiaphragm in said case and closing said open end, said case having anelongated groove in one surface, and a closure member over said groove,said member and said case containing passages for connecting theinterior of the oase and the atmosphere through said groove.

5. An acoustic device comprising a hollow enclosure having an open end,a diaphragm closing said open end, said enclosure having a groove in onesurface, and a closure member over said groove, said member andenclosure containing passages of substantially the same area as thecross-sectional area of the groove for connecting the interior of theenclosure and the atmosphere through said groove.

6. An acoustic device comprising a hollow enclosure having an open end,and a diaphragm closing said open end, said enclosure comprising a wallcontaining an elongated, restricted sound wave passage and having anaperture connecting the passage with the atmosphere outside theenclosure and another aperture connecting the passage with the interiorof the enclosure on the inner side of the diaphragm.

7. An acoustic device as claimed in the preceding claim in which theeffective length of said passage is variable.

8. An acoustic device comprising a hollow enclosure having an open end,a diaphragm closing said open end, said enclosure comprising a wallcontaining an elongated, restricted sound wave passage and having anaperture connecting the passage with the atmosphere outside of theenclosure and another aperture connecting the passage with the interiorof the enclosure on the inner side of the diaphragm, a partition memberin said` enclosure dividing it into two chambers with one of which saidpassage connects, said partition `member containing a passage connectingsaid chambers, and means for closing said last-mentioned passage.

9. An acoustic device comprising a hollow case having an open end, anelectrodynamic unit in said case and including a diaphragm closing saidopen end but exposed to sound waves therethrough, means for varying theeffective volume of the case on the rear side of the diaphragm, meansfor varying at will the response of the device at the lower end of theaudio frequency range, and means to vary at will the response of thedevice at the upper end of the audio frequency range.

10. An acoustic device comprising a hollow case having an open end, asound translating unit in said case and including a diaphragm closingsaid open end and being exposed therethrough to sound waves on one side,said case having a groove in its outer surface connected through anaperture in the case with the interior of the case on the other side ofthe diaphragm, and a closure member on said case and disposed over saidgroove, said member having an aperture therein connecting the groovewith the atmosphere outside the case and being slidable relative to thecase to vary the distance between the two apertures.

11. An acoustic device comprising a hollow case having an open end, asound translating unit in said case and including a diaphragm closingsaid open end and being exposed therethrough to sound waves on one side,said case having a groove in its outer surface connected through anaperture in the case with the interior of the case on the other side ofthe diaphragm, and a closure member on said case and disposed over saidgroove, said member having an aperture therein connecting the groovewith the atmosphere outside the case and being slidable relative tothecase to vary the distance between the two apertures, and means to holdsaid closure member against movement after adjustment of the closuremember for the desired spacing of the aperture.

12. An acoustic device comprising a hollow case having open ends, adiaphragm within the case in alignment with and closing one of said openends, a coil attached to said diaphragm, magnet structure for said coil,said case including a cover member for its other end and supporting anelongated open ended tubular member in the case, one open end of thetubular member being in alignment with an apertured portion of thecover, and a single wall member in said case dividing it into twochambers and supporting the magnet structure, diaphragm and coil in onechamber, said wall member having a restricted aperture dening a passageconnecting said chambers.

13. An acoustic device comprising a hollow case having an open end, asound wave translating unit in said case and including a diaphragm, adisc partition member slidably insertable in said case and dividing itintotwo chambers and having its central portion only bearing againstsaid unit to hold the latter in position so that the diaphragm closesthe case open end, means engaging the interior surface of said case andbearing against the peripheral portion of said partition member tomaintain the latter in position, said partition member containing asmall area, off-center passage connecting said chambers, and meansindependent of said partition member constituting a restricted soundwave passage for enhancing the low frequency response of the device,connecting one of said chambers with the atmosphere outside of the case.

14. An acoustic device comprising a case having open ends, a diaphragmwithin the case closing one open end, a coil attached to said diaphragm,a magnet structure for said coil, said case including a cover member forits other end and supporting an elongated open ended tubular member inthe case, one open end of the tubular member being in alignment with anapertured portion of the cover, and a wall member in said case dividingit into two chambers connected by a passage in the wall member, one ofsaid chambers containing the diaphragm, coil and magnet structure andthe other chamber containing said tubular member.

15. Anacoustic device comprising a hollow case having a substantiallyhemispherical section having an apertured Wall portion extending acrossits region of greatest diameter, and containing a sound wave passingopening in its curved surface portion, a diaphragm closing said opening,a cover member dening an air chamber with said wall portion, said membercontaining a passage ,connecting the chamber with the air outside thecase, and a tubular member between said wall portion and cover memberconstituting a continuation oi the passage.

16. An acoustic device comprising a casing having openings at its frontand back ends, a transmitter including a diaphragm and a magnet disposedwithin said casing, said diaphragm being positioned adjacent the frontend of said casing, an apertured partition within said casing betweensaid magnet and the back end of the casing, and a tubular memberentirely within` the chamber nearest the back end of the casing andcooperating with an opening in the rear of the casing to acousticallyconnect they open air with the back side of the diaphragm solely via theapertured partition.

17. An acoustic device comprising a casing having an opening at itsfront and back ends, an apertured partition within said casing, dividingthe interior into two chambers, a transmitter unit disposed in thechamber adjacent the front end of the casing, a tubular sound passagewaymember disposed in the chamber adjacent the back end of the casing andsupported thereon, and means on said partition for holding said unit inplace.

ROBERT N. MARSHALL.

